(1) Field of the Invention
The present invention relates to flexible coupling means that can tolerate angular and axial shifts while connecting together two rotary mechanical units. The coupling means may for example connect a power transmission shaft to a part or a mechanical assembly such as a turboshaft engine of a rotorcraft. The invention also relates to a mechanical transmission provided with the flexible coupling means, and to an aircraft.
Consequently, the invention lies in the technical field of transmitting power from one mechanical unit to another mechanical unit, and in particular mechanical units of a rotorcraft.
(2) Description of Related Art
In particular, most presently-constructed rotorcrafts have at least one turboshaft engine with a free turbine. Power is then taken from a low pressure stage of the free turbine, which stage is mechanically independent of the compressor assembly and of the high pressure stage of the engine. The free turbine of a turboshaft engine has a speed of rotation that generally lies in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so it is necessary for a speed-reducing gearbox to be provided in order to connect with the main rotor of the rotorcraft, since its speed of rotation generally lies in the range 200 rpm to 400 rpm: this power transmission gearbox is commonly referred to as the main gearbox (MGB).
Under such conditions, the engine is connected to the main gearbox of the rotorcraft via at least one transmission shaft rotating about its axis of rotation at a speed that is often greater than 5000 rpm.
Likewise, a rotorcraft may include a tail rotor driven by at least one power transmission shaft for controlling yaw movement of the rotorcraft. The speed of rotation of the tail rotor is likewise less than the speed of rotation of the free turbine.
In addition, it should be recalled that the power transmitted by an element rotating about an axis is equal to the product of the speed of rotation of that element multiplied by the mechanical torque applied to said element.
In general manner, a power transmission shaft needs to be securely fastened by coupling means to the mechanical units that are to be set into rotation. Such a power transmission shaft is referred to below as a “transmission shaft”, for short.
Furthermore, the coupling means must enable the transmission shaft to transfer the power developed by one mechanical unit to another mechanical unit under conditions that are extreme, i.e. when the interconnected members are not properly aligned with each other.
Means for coupling a transmission shaft to a mechanical unit then need to be dimensioned to accommodate axial and angular misalignment between the transmission shaft and the mechanical unit.
Known mechanical coupling means accommodating such misalignments are provided with a first annular member and with a second annular member.
Each annular member has a thin flexible annular disk extending radially from a base to an outer periphery. The base is secured to means for fastening to a rotary unit, such as a transmission shaft.
Each annular disk is sometimes referred to as a “diaphragm”. The term “diaphragm” is therefore used below for reasons of convenience.
Under such circumstances, the two annular members are connected together solely via the outer peripheries of the diaphragms. The outer peripheries may be fastened together by welding, or indeed using nuts blocked by bolts, for example.
Diaphragm coupling means provide an advantageous solution for interconnecting rotary units that might be out of alignment.
Such diaphragm coupling means possess a relatively high level of reliability because of the small number of parts. Nevertheless, diaphragm coupling means with thin diaphragms can lead to a degree of weakness on going beyond the nominal axial and angular misalignments that were taken into account when they were designed.
Document U.S. Pat. No. 5,588,917 describes connection means having two torque transmission paths, one of the paths including fluting.
Document FR 2 990 011 describes coupling means having a first member suitable for being fastened to a first rotary unit and a second member suitable for being fastened to a second rotary unit. The first member has a first diaphragm, the second member being provided with a second diaphragm secured to the first diaphragm. An emergency torque transmission device has at least one bayonet system including a protrusion secured to one of the members co-operating with an angled groove formed in the other member, and being inserted into the angled groove by a push-and-turn movement, the coupling means, in the absence of rupture, leaving axial clearance both in compression and in traction, and circumferential clearance between each protrusion and walls defining the corresponding angled groove.
Documents FR 2 449 231, U.S. Pat. No. 4,133,188, JP H02 53527 U, and FR 1 426 477 are also known.